Process for producing pure ammonium metallates

ABSTRACT

The present invention relates to a process for the production of pure ammonium metallates of the metals W, Mo, V, Cr and Re, wherein alkaline metal salt solutions are adjusted to a pH value in the range from 5 to 9 by reaction with CO 2 , these metal salt solutions are passed through an anion exchanger in carbonate form, wherein, in addition to the alkali metal carbonate solution, the metallate form of the ion exchanger is formed, the ion exchanger is then washed until free of alkali and is then eluted by means of aqueous ammonia to form ammonium metallate solutions and the anion exchanger in the OH form, wherein the anion exchanger is converted into the carbonate form by means of CO 2  treatment and is recycled.

The present invention relates to a process for the production of pure ammonium metallates of the metals W, Mo, V, Cr and Re, wherein alkaline metal salt solutions are adjusted to a pH value in the range from 5 to 9 by reaction with CO₂, these metal salt solutions are passed through an anion exchanger in carbonate form, wherein, in addition to the alkaline metal carbonate solution, the metallate form of the ion exchanger is formed, the ion exchanger is then washed until free of alkali and is then eluted by means of aqueous ammonia to form ammonium metallate solutions and the anion exchanger in the OH form, wherein the anion exchanger is converted into the carbonate form by means of CO₂ treatment and is recycled.

In order to produce pure ammonium metallates, in particular of the refractory metals W, Mo and V as precursors for recovery of the metals, the raw materials are generally treated with alkaline NaOH or Na₂ CO₃ solutions. An excess of alkali is used in this operation in order to achieve the greatest possible yields of the valuable metal. In the subsequent processing stage, the pH value of these solutions must be reduced. Since this is generally achieved by using mineral acids, this inevitably results in the formation of neutral salts, which must be disposed of.

Once the alkaline metallate solutions obtained in this manner have been digested and purified, they are converted into the ammonium metallates.

This is principally achieved by solvent extraction processes (Ulmanns Encyclopadie der technischen Chemie, 4th edition, volume 24, 1983, page 466) or recovery using solid ion exchangers (Uranium Mining and Metallurgy, volume 10(3), August 1991).

The reduction in pH is also achieved in these two processes by the addition of mineral acids, inevitably resulting in the formation of considerable quantities of neutral salts which must be disposed of.

An improved solvent extraction process is disclosed in SU-A 1 792 993 and SU-A 1 792 994 (Derwent 94-133033/16 and 94-133084/16). According to this process, acidification is performed with CO₂, resulting in the formation of Na₂ CO₃, which may be returned to the process. In the process described, the metallate anion is converted into an organic phase and then only partially re-extracted with NH₃ solution. However, up to approx. 50% of the stoichiometric quantity of the extracting agent remain blocked by the metallate anion.

The object of the present invention is to provide a process which avoids environmental pollution by the neutral salts and the disadvantages of incomplete utilisation of the extracting agent.

This object is achieved by a process for the production of pure ammonium metallates of the metals W, Mo, V, Cr and Re, wherein alkaline metal salt solutions are adjusted to a pH value in the range from 5 to 9 by reaction with CO₂, these metal salt solutions are passed through an anion exchanger in carbonate form, wherein, in addition to the alkaline metal carbonate solution, the metallate form of the ion exchanger is formed, the ion exchanger is then washed until free of alkali and is then eluted by means of aqueous ammonia to form ammonium metallate solutions and the anion exchanger in the OH form, wherein the anion exchanger is converted into the carbonate form by means of CO₂ treatment and is recycled, wherein the anion exchanger used is a solid anion exchanger.

The solid anion exchanger used is preferably a moderately basic ion exchanger. These are those ion exchangers which contain tertiary and quaternary ammonium groups as the active components. Better results are achieved with weakly basic ion exchangers, i.e. those predominantly containing tertiary ammonium groups as the active components. These may be almost completely eluted with aqueous NH₃.

The best results are achieved if the alkaline metal salt solutions are reacted with CO₂ at pressures ranging from standard pressure to 50 bar, preferably from standard pressure to 10 bar.

The exchanger may also advantageously be laden with CO₂ by means of CO₂ /water at pressures of 1 to 10 bar. In this manner, it is simple to convert fixed bed exchangers in particular into the carbonate form.

Elution of the anion exchanger by means of aqueous ammonia is preferably performed with a concentration of 5 to 20 wt. % of NH₃ up to a degree of exchange of at least 90%.

A particularly advantageous variant of the process according to the invention consists in concentrating the raffinate containing the alkaline metal carbonate, recycling it and using it to digest metal raw materials for the production of the alkaline metal salt solutions.

The course of the process is illustrated below using tungsten by way of example, but this should not be considered to limit it in any way.

The sodium alkaline digestion solution is treated with gaseous CO₂ until a pH value of 8.5 is reached. P, As and Si impurities are separated at this pH using known precipitation processes. After the subsequent filtration, the solution is diluted to 20 to 50 g of WO₃ /l and adjusted to a pH value of approx. 7 with CO₂.

The exchanger plant, which comprises two or more columns, is filled with a moderately to weakly basic anion exchanger. The exchanger resin is converted into the CO₃ ²⁻ form by an aqueous suspension with CO₂, preferably under pressure. The adjusted tungstate solution passes through the exchanger columns in succession. The pH value is readjusted between the individual columns by addition of CO₂. A solution containing 15 g of Na₂ CO₃ /l leaves the final column. Once the first column is laden with WO₄ ²⁻, the column is washed with H₂ O until free of sodium, eluted with dilute NH₃ solution at temperatures of 50° to 80° C., rewashed, reformed with CO₂ and switched to the end of the plant. The eluate may be worked up using known processes.

Compared with solvent extraction, the process according to the invention may be performed using simpler plant and equipment and is easier to control.

The invention is illustrated below by way of example, but this should not be considered to limit it in any way.

EXAMPLE 1 (Comparative Example according to SU-A 1792993/4)

Continuous extraction of W using CO₂ to establish the required pH value was investigated using a laboratory SX plant (mixer/settler).

The Na₂ WO₄ solution used was diluted to 70 g/l of W and treated with gaseous CO₂ until a pH value of 7.0 was established. The organic phase (15% Adogen 464, 8% isodecanol, remainder Escaid 120), which was circulated, was also treated with gaseous CO₂ in the storage vessel.

Once the organic phase (OP) was laden with 35 to 40 g/l of W, approx. 55% of the W loading could be stripped with NH₃ solution. The remaining 45% remained on the circulating OP or could only be isolated with NaOH as an Na₂ WO₄ strip.

EXAMPLE 2 (according to the invention)

Various weakly, moderately and strongly basic anion exchangers were tested in order to determine the optimum anion exchanger for recovery of ammonium paratungstate (APT). All the exchangers were completely laden with excess Na₂ WO₄ solution, the excess Na₂ WO₄ was washed out and the bound WO₄ ²⁻ eluted with 10% NH₃ solution. Only the weakly basic anion exchangers were completely elutable. In the case of the moderately basic anion exchangers, 10 to 20% of the W loading remains on the anion exchanger, depending upon the content of quaternary ammonium groups. In the case of the strongly basic anion exchangers containing only quaternary ammonium groups, only less than 1% to 45% of the W loading could be eluted with NH₃ solution. The remaining WO₄ ²⁻ could only be eluted with NaOH.

The test conditions for Example 2 are shown in the following table.

    ______________________________________                        Deliv-        NH.sub.4                                            Capacity     Anion              ery     pH    eluate                                            g of W/l of     exchanger Basicity form    loading                                      g/l of W                                            exchanger     ______________________________________     Lewatit.sup.1)  AP 29               moderate OH      8     13.5  135     Lewatit AP 246               weak     OH      8     12.9  129     Lewatit MP 62               weak     OH      8.5   18.7  187     Lewatit MP 64               moderate OH      8.5   16.4  164     Lewatit MP 500               strong   Cl      8.5   3.35  33.5     Lewatit MP 600               strong   Cl      8.5   7.6   76     Lewatit M 500               strong   Cl      8     0.08  0.8     Lewatit M 504               strong   Cl      8     0.35  3.5     Lewatit M 600               strong   Cl      8     0.145 1.45     ______________________________________      .sup.1) Commercial product of Bayer AG, Leverkusen, Germany

EXAMPLE 3

Loading tests with various refractory metals were performed using the weakly basic exchanger Lewatit MP 62, which according to table 1 exhibits the highest loading capacity elutable with ammonia solution. The exchanger was formed with CO₂ at standard pressure. Loading was performed in each case with identical quantities, calculated as W, relative to a 20 g/l sodium tungstate solution. Elution was performed with a 1:1 diluted aqueous NH₃ solution.

The results are shown in table 2 below.

                  TABLE 2     ______________________________________     Metal      pH        Loading     ______________________________________     W          7         153.5  g of W/l of exchanger     Mo         6-6.5     51.2   g of Mo/l of exchanger     Cr         6-6.5     40     of Cr/l of exchanger     V          7         75.7   g of V/l of exchanger     Re         7         107    g of Re/l of exchanger     ______________________________________

Absorption capacity could be raised by 20% by treating the exchanger Lewatit MP 62 with gaseous CO₂ at 10 bar in comparison with gaseous CO₂ treatment at standard pressure. 

We claim:
 1. Process for production of pure ammonium metallates of a metal selected from the group consisting of W, Mo, V, Cr and Re, comprising the steps of:(a) providing an alkaline metal salt solution of the metal, (b) reacting the solution with CO₂ to adjust the pH of the solution within a range of 5 to 9, (c) contacting the pH adjusted solution with a carbonate form solid anion exchange material to produce a metallate form of the anion exchange material and an alkaline metal carbonate solution, (d) eluting the metallate form of the anion exchange material with aqueous ammonia to form (i) an ammonium metallate solution and (ii) an OH form of the anion exchange material, (e) separating the thus formed ammonium metallate solution and OH form anion exchange material and treating the OH form anion exchange material with CO₂ to convert it to a carbonate form and recycling the carbonate form anion exchange material to step (c) for use as the carbonate form solid anion exchange material of step (c).
 2. Process according to claim 1, wherein the solid anion exchange material is a moderately basic ion exchanger.
 3. Process according to claim 1, wherein the solid anion exchange material is a weakly basic ion exchanger predominantly containing tertiary ammonium groups.
 4. Process according to any of claims 1, 2, and 3, wherein the reaction of the alkaline metal salt solutions with CO₂ is performed at pressures ranging from standard pressure to 50 bar.
 5. Process according to claim 4, wherein the OH form anion exchange material is converted into the carbonate form with CO₂ /water at pressures of 1 to 10 bar.
 6. Process according to claim 5, wherein the metallate form of the anion exchange material is eluted with aqueous ammonia of a concentration of 5 to 20 wt. % NH₃ to a degree of exchange of at least 90%.
 7. Process according to claim 6, wherein the alkali metal carbonate solution is concentrated, recycled and then metal raw materials are digested to produce the alkaline metal salt solution.
 8. Process according to claim 4 wherein the metallate form of the anion exchange material is eluted with aqueous ammonia of a concentration of 5 to 20 wt. % NH₃ to a degree of exchange of at least 90%.
 9. Process according to any of claims 1, 2, and 3 wherein the OH form anion exchange material is converted into carbonate form contacting with CO₂ /water at pressures of 1 to 10 bar.
 10. Process according to any of claims 1, 2, and 3 wherein the alkaline metal carbonate solution is concentrated and recycled and then metal containing raw materials are digested thereby to produce the alkaline metal salt solution. 